Abstract

Trace-element geochemistry on 176 controlled samples in three adjoining Abitibi (∼2.7 b.y.) volcanic piles confirms the fundamental concept of Archean volcanic cycles composed of lower tholeiitic and upper calc-alkalic parts. The volcanic piles, each as much as 16 km thick, are either unicyclic or multicyclic. In the latter case, the cycles display secular geochemical trends. All the volcanic rocks studied are indicated to have come from mantle sources. A fundamental feature of Abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources. Abitibi tholeiitic basalts closely resemble modern mid-ocean ridge basalts. Abitibi calc-alkalic andesites and alkalic rocks are very similar to modern oceanic island-arc andesites and to some modern volcanic-arc high-K rocks, respectively. The principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism—a type of “hot-spot tectonics” mechanism involving a layered mantle responsible for the early tholeiitic (depleted mantle source) and later calc-alkalic (undepleted mantle source) parts, respectively, of the volcanic cycles.

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